Cross reference is made to the following co-pending patent applications, each being assigned to the same assignee as the present invention and the teachings included herein by reference:
The present invention is generally related to the handling of semiconductor wafers, and more particularly to a method for securely adhering fragile wafers to a wafer tape including wafers that are already partially sawn.
In the manufacture of semiconductor devices, it is conventional to form many integrated circuits or devices upon a single wafer of material, such as silicon. After the devices have been formed on the wafer, it is necessary to separate each device from one another such as by completely cutting the wafer into segments on which one or more devices or circuits have been formed, these segments commonly being referred to as die. For conventional integrated circuits, the completed wafer is protected with an overcoat of material, such as silicon dioxide. The completed wafer can then be securely adhered to an adhesive wafer tape stretched across a circular wafer frame, whereby the active wafer surface can be physically supported by a surface. Rollers or wheels may be rolled across the backside of the tape proximate the supported wafer to secure the tape to the wafer. The protective overcoat protects the active devices from damage during the physical application of force. The wafer can be completely sawn along the streets separating the individual circuits to form the die. The die remain securely adhered to the wafer tape, and are later removed by pick and place equipment for die packaging.
For devices that are unsuitable for a complete saw process, due to fragile elements on the surface of the wafer, a partial saw process may be performed. Such devices that are unsuitable for a complete saw process are micromechanical devices having moving parts including the digital micromirror device (DMD) manufactured by Texas Instruments of Dallas, Texas. Orthogonal lines may be scribed on the wafer, or a partial saw can be formed along the wafer streets extending between devices, the formed saw lines commonly being known as kerfs. A domed wafer is pressed against the wafer backside to break the wafer along these wafer kerfs to form individual die. One such method is taught in commonly assigned Ser. No. 08/975,378 entitled xe2x80x9cMethod and Apparatus for Breaking and Separating Die from a Wafer using a Multi-Radii Domexe2x80x9d, and Ser. No. 08/975,029 entitled xe2x80x9cMethod and Apparatus for Stretching Saw Film Tape after Breaking a Partially Sawn Waferxe2x80x9d, the teachings of which are incorporated herein by reference. If care is not taken during the breaking of the wafer, the individual die may be partially fractured or broken.
As illustrated in these cross referenced patent applications, the wafer break process is facilitated by placing the fabricated wafer upon a stretchable membrane such as wafer tape having an adhesive on one side. As the wafer is broken with an anvil, the tape is stretched to further separate the formed die from one another to avoid the die corners from rubbing against one another. After the wafer break process, pick and place equipment removes the individual die from the tape. These die are then packaged with leads and pins, the packaging comprising plastic, ceramic or other suitable material. Sometimes, the die are hermetically sealed in the package to prevent moisture from damaging the device, particularly if the device is a micromechanical device having moving elements.
It is important that the fabricated wafer is securely placed upon the stretchable membrane to achieve an effective wafer break, especially if the wafer is inverted during the wafer break process. It is preferred to invert the wafer during the wafer break such that any particles generated during the break fall downwardly and away from the fragile micromechanical devices. If the wafer is not sufficiently secured to the tape, the individual die may fall off the tape and be scraped. Any air bubbles need to be removed from between the tape and wafer to avoid die shifting and engaging one another.
There is disclosed in commonly assigned U.S. Pat. No. 6,007,654 entitled xe2x80x9cNoncontact Method of Adhering a Wafer to a Wafer Tapexe2x80x9d a noncontact method for securely adhering a fragile wafer to a wafer tape that is already partially sawn. Specifically, there is disclosed a noncontact method using a compressed gas to firmly attach the wafer tape to the wafer.
It is desired to provide an improved method and apparatus for securely adhering fragile semiconductor wafers to a wafer tape, particularly partially sawn wafers that are to be broken along kerfs to form individual die. It is especially desired to provide an improved method for adhering fragile wafers to an adhesive wafer tape that have fragile active surfaces which can not be physically pressed upon a support surface while securing the wafer to the tape. The fragile wafer needs to be securely adhered to the wafer such that it can be inverted, allowing particles generated during wafer break to fall downwardly without risk of the die inadvertently releasing from the wafer tape.
The present invention achieves technical advantages as a contact method and apparatus which applies a gentle concentrated point force to a backside of a wafer tape proximate a supported wafer, thereby securely adhering an opposing adhesive side of the tape to the wafer. The gentle point force is selectively applied with a roller to the wafer tape, as it is supported taut across a saw frame, with sufficient pressure to cause the sticky saw film to adhere to the back of the fragile semiconductor wafer, without causing it to break. Application of the roller pressure is a contact method which eliminates the necessity for physically handling or supporting the active surface of the wafer, such as a wafer having micromechanical devices. The present invention is especially suitable for mounting fragile wafers that are partially sawn and which are eventually separated by pressing an anvil against the back side of an inverted wafer.
According to the preferred method of the present invention, a concentrated point force is first directed upon and focused against the wafer tape proximate the center of the tape. The wafer is placed upon the protruding point and held in place by the force of gravity. The gentle point force is then rotated by a turntable and advanced outwardly below the wafer and wafer tape, thereby defining a spiral or helix pattern to securely adhere the wafer tape to the wafer without introducing air bubbles between the wafer tape and wafer. The width of the point force is sufficient to sweep a spiral path that overlaps the previous path. Preferably, there is a 20%-50% overlap of the spiral path to ensure any gas between the tape and wafer is directed radially outward and does not become entrapped between the tape and wafer. The point force pressure is selected such that the wafer does not break.